1. Field of the Invention
This invention relates to a method for producing a KTiOPO.sub.4 single crystal employed as a non-linear optical material.
2. Description of the Prior Art
With the advent in recent years of a laser having a strong output and excellent coherence, it has become possible to produce a light having a wavelength one-half the basic wavelength, by way of second harmonic generation (SHG), with the use of a non-linear optical material.
Typical of the non-linear optical material is a KTiOPO.sub.4 single crystal, which is a non-linear optical crystal, referred to hereinafter simply as KTP.
For producing the KTP, there are known a hydrothermal growth method and a top seeded solution growth method (TSSG method). However, in any of these methods, multiple domains are produced on the KTP single crystal produced by growth as a result of polarization.
These multiple domains, if present, give rise to a lowered output and a lowered efficiency of the non-linear optical material in respect of SHG, as described in Appl. Phys. Lett., 51, (1987), 1322.
For overcoming these difficulties, attempts have been made of heat-treating the bulk material or substrates, sliced from a crystal ingot presenting the multiple domains, or applying an electrical field to the bulk material or substrate during the heat treatment for converting the multidomain state into the single domain state. For example, the above-cited literature discloses a poling method of slicing the grown multidomain single crystal in a direction at right angles to the C-axis (polarization axis) to produce a C-plate, forming electrodes on front and reverse sides of the C-plate and applying an electrical voltage across the front and reverse sides of the C-plate heated to about 500.degree. C. to produce a single-domain crystal.
However, these known methods suffer from drawbacks that the single domain crystal can be produced only with difficultly depending on the type of multidomain arrays, such as an array presenting a zigzag-shaped domain wall, the poling process inclusive of an electrode formation is complex and defects produced at the domain boundary tend to remain after poling. As a matter of fact, experiments conducted in converting the multidomain crystal into the single domain crystal by heat treatment and application of the electrical field during the heat treatment have revealed that inversion of polarization cannot be achieved at certain sites and hence a satisfactory single-domain crystal cannot be produced.